Home | Session 3 | Sedimentary Rock Pg 1, 2, 3, 4, 5, 6, 7, 8, 9
What's in a Sedimentary Rock?
Presented
by Carol Tang
California Academy of Sciences
Introduction
There are many "stories" hidden with sedimentary rocks. Close examination can reveal the source of the rock, the processes at work during its formation, and even what was going on in the environment. Clues include composition, color, grain size, and angularity of the grains. For instance, crystals that remain highly angular indicate little tumbling and perhaps transportation by air, while very rounded grains may indicate transportation by fast moving water.
Geologic features such as sand dunes, ripple marks, or mud cracks are familiar to us, but we can also find ancient versions of these features preserved within the rocks. Uniformitarianism - the hypothesis that current geologic processes have been occurring in a similar manner throughout geologic history - allows us to understand the patterns in the rocks.
The colors in the rocks tell us about the environment during their formation. Red rocks (iron oxide) are typical of deposition on land during dry periods. While the dark clays in England indicate rich organic materials. We can also see changes in sea levels.
The fossils within the rocks also tell us stories - "who" was there and what was happening to them.
Enjoy this slide show presented by Carol Tang and see if you can find some of the stories told by the sedimentary rocks.
Goosenecks, northern Arizona. Slide shows modern river cutting through sedimentary rocks which are about 150 million years old. Slide illustrates recycling of materials for sedimentary rocks. Thin section through Jurassic-age cobble (Sundance Formation, Wyoming). The light colored grains are quartz grains. By looking at the shape and size of the grains, we can often tell the history of the grains. For example, many of these grains are angular, which suggests that they were deposited near their source. (Slide is 2.6 mm across)
updated February 11, 2002
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